1. Field of the Invention
The present invention relates to an inkjet printing apparatus capable of discharging liquid by applying energy to the liquid, and a method of driving a printhead used in the inkjet printing apparatus.
2. Description of the Related Art
Inkjet printing apparatuses are mainly used to print photographs and postcards, and have advantages of high-speed printing, high quality, low noise, and printing on a variety of media. Along with recent popularization of digital cameras and personal computers, the market of inkjet printing apparatuses is growing rapidly. The range of use of the inkjet technique is becoming wider such that the inkjet technique of discharging a predetermined amount of droplet and attaching it to a print medium is exploited in the industrial field. Along with this, the printhead used in the inkjet printing apparatus is improving its performance more and more, and the technical innovation is accelerating.
There are mainly known two types of inkjet methods, that is, a Bubblejet® method and piezoelectric method. The Bubblejet® method is a method of applying heat energy to ink to change the ink state accompanied by an abrupt change of volume (generation of bubbles), and discharging ink from an orifice by a force generated based on the state change. The piezoelectric method is a method of applying a voltage to electrodes on the two surfaces of a piezoelectric element to deform the piezoelectric element, and discharging ink from an orifice by the volume change.
By using any of these methods, inkjet printing apparatuses form an image by attaching ink discharged from an orifice onto a print medium.
Since inkjet printing uses ink whose main component is water, the viscosity of ink increases upon evaporation of water or the like, and a discharge failure and clogging readily occur. To avoid the discharge failure and clogging, the orifice is refreshed by executing discharge (preliminary discharge) irrelevant to ink discharge for printing an image before starting the printing operation.
Recently, to meet market needs for higher-resolution images and higher-speed printing for inkjet printing apparatuses and expectation of application of inkjet printing apparatuses to industrial uses, a technique for stably discharging a smaller droplet than the conventional one has been developed. Also, a technique for achieving an objective of suppressing a satellite droplet, which is generated after a main droplet and is smaller than the main droplet has been developed.
Satellite droplets cause various problems. For example, an ink droplet of a smaller particle diameter is more susceptible to the influence of air resistance. Thus, under the influence of an air flow generated when a main droplet passes through air, a subsequent satellite droplet might attach to an unintended portion on a print medium, degrading the image quality. Further, satellite droplets of extraordinarily small particle diameter do not attach to a print medium, but float as ink mist and contaminates the interior of the apparatus.
In preliminary discharge, a print medium is often not fed to an ink discharge position, and ink droplets are readily influenced by air resistance. It is known that the amount of floating mist tends to become larger in preliminary discharge than in printing. As a measure against floating mist, it is effective to decrease mist generated in preliminary discharge.
Japanese Patent Laid-Open No. 4-239649 discloses a technique of controlling driving of a printhead in preliminary discharge. More specifically, Japanese Patent Laid-Open No. 4-239649 discloses a technique of driving a printhead at a driving frequency which changes over time or a driving frequency equal to or higher than that in printing, setting a process of making the ink meniscus of an orifice convex, and removing ink attached to the periphery of the orifice. However, this technique aims to removing ink attached to the periphery of an orifice, and does not decrease ink mist in preliminary discharge.
The present inventors have found that it is possible to change the ink meniscus of an orifice from the convex state to the concave state by changing the driving time interval between adjacent nozzles to generate crosstalk between them. The inventors have also found that as crosstalk between adjacent orifices changes the ink meniscus of the orifice to the convex or concave state, the satellite droplet formation state also changes. The inventors have made extensive studies to find that generation of satellite droplets is greatly reduced by driving nozzles and starting the discharge operation when the ink meniscus of the orifice becomes convex. The inventors have also found that the generation of satellite droplets increases by driving nozzles and starting the discharge operation when the meniscus of the orifice becomes concave.
To reduce ink mist from the above extensive studies, it is effective to drive nozzles and start the discharge operation when the ink meniscus of the orifice becomes convex.
As described above, floating ink mist can be reduced by adjusting the driving time interval between adjacent nozzles, and when the ink meniscus of the orifice becomes convex, driving nozzles and starting the discharge operation. However, the state in which crosstalk occurs is an unstable state, and crosstalk is likely to influence printing. To prevent degradation of the image quality, it is generally known that the nozzle must be driven to start the discharge operation while the influence of crosstalk is minimized. In short, to prevent degradation of the image quality, the driving time interval between adjacent nozzles is preferably as large as possible. To the contrary, to make the ink meniscus of the orifice convex in order to reduce floating ink mist, the driving time interval between adjacent nozzles needs to be set to a predetermined value or smaller. It is revealed that the driving time interval between adjacent nozzles for preventing degradation of the image quality and that for reducing floating ink mist have a trade-off relationship.
That is, if the driving time interval between adjacent nozzles is set large in order to prevent degradation of the image quality, this causes a problem that floating ink mist increases in preliminary discharge and contaminates the interior of the apparatus. On the contrary, if the driving time interval between adjacent nozzles is adjusted to an interval at which crosstalk easily occurs in order to reduce floating ink mist, this causes a problem that degradation of the image quality cannot be prevented.